Canada Thistle: Options for Control

Canada Thistle (Cirsium arvense) is a Class “C” Noxious Weed (Control is encouraged in vehicle corri-dor areas of large infestation and required in areas of limited distribution). Canada thistle is a perennial herb with a deep-seated complex system of roots spreading horizontally which give rise to aerial shoots. The 1 to 4 foot tall stems are slender, green, and freely branched. The leaves are alternate, sessile, and deeply lobed. The leaf margins have stiff yellowish spines. The heads are many and relatively small. The flowers are purple. The fruits are about 1/8 inch long, somewhat flattened and brownish with an apical circle of ling hairs, these even-tually falling. Four varieties of C. arvense have been recognized based on variation in leaf characters, tex-ture, vestiture, segmentation, and spinyness. Canada thistle can grow on a wide variety of soil types; clay loam, sandy loam, sandy clay, and sand dunes. It does poorly on wet soils without much aeration. It is found in almost every plant community disturbed by man. It is common to roadsides, railway embankments, lawns, gardens, abandoned fields, sand dunes, agricultural fields, margins of forests, and waterways. It grows poorly in shaded conditions and produces few flowers. Canada thistle is mainly insect pollinated. Many insect visitors have been reported. The average seed produc-tion is about 1,530 seeds per plant, but exceptional plants may produce up to 5,300 seeds.

The best invasive weed control is

prevention!

Economic Importance Beneficial: Canada thistle is regarded as a good source of nectar and pollen for honey bees. The shoots and roots are consumed by some people in Russia and by North American Indians. Detrimental: Canada thistle is found in virtually all crops, and is also found in pastures and rangelands where it reduces forage. The weed is an effective competitor for light, moisture, and nutrients and is thus able to reduce crop yields. It also serves as an alternate host for insects and pathogenic microorganisms that attack various crops. History: Canada thistle was probably introduced to North America by early colonists in the 17th Century. Control legislation was enacted in Vermont in 1795 and by New York in 1831. It was not reported west of the Allegheny Mountains until after 1835.

Canada Thistle in Bloom

Canada Thistle flowers

Canada Thistle

Population Dynamics: By vegetative propagation, a single seedling can establish a large patch of stems. However no seeds will be produced because seed production requires the presence of both sexes. More than on introduction is thus needed. Plant competitors vary in their effectiveness. In one study, the percent-age of thistles increased by 192 percent in 4 years in a continuous cropped spring wheat, but decreased to 1% in alfalfa grown for hay. Response to Herbicides: Effective control can be achieved by using several broad-leaved herbicides that do not harm grasses. For more site specific control recommendations, please refer to the latest edition of the Pacific Northwest Weed Control Handbook. Response to Cultural Methods: Planting competitive crops, such as alfalfa and forage grasses can be very effective in controlling an infestation of Canada thistle.

Ferry County Noxious Weed Control Board 350 E. Delaware Ave., #14 Republic, WA 99166 Phone: (509) 775-5225 x1111 Fax: (509) 775-5218

Canada Thistle

Canada thistle Cirsium arvense

Canada thistle is a Class “C” Noxious Weed Control is encouraged in vehicle corridor areas of large infestation and required in areas of limited distribution.

For additional information about weed identification and control, contact:

Prevention Early detection is vital to prevent invasion. Prevent spread from infested fields by cleaning equipment.

Canada thistle spreads by seeds, horizontal roots that produce shoots, and by root fragmentation. Dense colonies impact cultivated crops and pastures.

Canada thistle infesting a field.

Photos by Eileen Sande and Mary Fee